U.S. patent application number 11/466624 was filed with the patent office on 2008-09-04 for emission control system.
This patent application is currently assigned to United States of America as Represented by the Administrator of the National Aeronautics & Space. Invention is credited to Landy Chung, Clyde F. Parrish.
Application Number | 20080213148 11/466624 |
Document ID | / |
Family ID | 37834104 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080213148 |
Kind Code |
A1 |
Parrish; Clyde F. ; et
al. |
September 4, 2008 |
Emission Control System
Abstract
Methods and apparatus utilizing chlorine dioxide and hydrogen
peroxide are useful to reduce NOx emissions, as well as SOx and
mercury (or other heavy metal) emissions, from combustion flue gas
streams.
Inventors: |
Parrish; Clyde F.; (Trinity,
FL) ; Chung; Landy; (Jacksonville, FL) |
Correspondence
Address: |
NASA JOHN F. KENNEDY SPACE CENTER
MAIL CODE: CC-A/OFFICE OF CHIEF COUNSEL, ATTN: PATENT COUNSEL
KENNEDY SPACE CENTER
FL
32899
US
|
Assignee: |
United States of America as
Represented by the Administrator of the National Aeronautics &
Space
Washington
DC
|
Family ID: |
37834104 |
Appl. No.: |
11/466624 |
Filed: |
August 23, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60737015 |
Nov 14, 2005 |
|
|
|
Current U.S.
Class: |
423/235 ;
422/168 |
Current CPC
Class: |
B01D 2251/108 20130101;
B01D 2251/106 20130101; B01D 53/56 20130101; B01D 53/507 20130101;
B01D 53/64 20130101; B01D 2251/506 20130101 |
Class at
Publication: |
423/235 ;
422/168 |
International
Class: |
B01D 53/56 20060101
B01D053/56; B01J 19/00 20060101 B01J019/00 |
Claims
1. A method of treating a gas stream for removal of at least oxides
of nitrogen, the method comprising: generating chlorine dioxide in
a continuous reaction process; mixing the chlorine dioxide with the
gas stream upon generation; maintaining a desired level of nitric
oxide in the gas stream by regulating a generation rate of the
chlorine dioxide; scrubbing the gas stream with a hydrogen peroxide
solution, thereby producing a first scrubber liquor; and
maintaining a desired hydrogen peroxide concentration in the first
scrubber liquor by regulating a flow of the first hydrogen peroxide
solution.
2. The method of claim 1, wherein generating chlorine dioxide in a
continuous reaction process comprises continuously adding a sodium
chlorate solution and a hydrogen peroxide and sulfuric acid
solution to a reaction vessel.
3. The method of claim 2, wherein mixing the chlorine dioxide with
the gas stream upon generation comprises removing the chlorine
dioxide from the reaction vessel using an eductor.
4. The method of claim 3, further comprising: passively adding
make-up air to the reaction vessel to maintain the reaction vessel
at ambient conditions.
5. The method of claim 2, wherein mixing the chlorine dioxide with
the gas stream upon generation comprises pressuring the chlorine
dioxide from the reaction vessel into the gas stream.
6. The method of claim 2, further comprising: adjusting a rate of
addition of the sodium chlorate solution and a rate of addition of
the hydrogen peroxide and sulfuric acid solution to regulate the
generation rate of the chlorine dioxide.
7. The method of claim 1, wherein scrubbing the gas stream with the
first hydrogen peroxide solution further comprises: feeding the gas
stream into a scrubber; feeding the first hydrogen peroxide
solution into the scrubber; and recycling the first scrubber liquor
into the scrubber with the first hydrogen peroxide solution.
8. The method of claim 7, further comprising: sampling a mixture of
the first hydrogen peroxide solution and the recycled first
scrubber liquor for a level of hydrogen peroxide; and controlling
the level of hydrogen peroxide in the mixture to be at least a
predetermined level.
9. A method of treating a gas stream for removal of at least oxides
of sulfur and oxides of nitrogen, the method comprising: scrubbing
the gas stream with a first hydrogen peroxide solution, thereby
producing a first scrubber liquor; maintaining a desired hydrogen
peroxide concentration in the first scrubber liquor by regulating a
flow of the first hydrogen peroxide solution; removing a by-product
stream containing sulfuric acid after attaining a desired sulfuric
acid concentration in the first scrubber liquor; after scrubbing
the gas stream with the first hydrogen peroxide solution, oxidizing
the gas stream using a chlorine dioxide mixture adapted to convert
nitric oxide to nitrogen dioxide; and after oxidizing the gas
stream, scrubbing the gas stream with a second hydrogen peroxide
solution.
10. The method of claim 9, wherein scrubbing the gas stream with
the first hydrogen peroxide solution further comprises: feeding the
gas stream into a scrubber; feeding the first hydrogen peroxide
solution into the scrubber; removing solids from the first scrubber
liquor; and recycling the first scrubber liquor into the scrubber
with the first hydrogen peroxide solution.
11. The method of claim 10, further comprising: sampling a mixture
of the first hydrogen peroxide solution and the recycled first
scrubber liquor for a level of hydrogen peroxide; and controlling
the level of hydrogen peroxide in the mixture to be at least a
predetermined level.
12. The method of claim 10, wherein removing solids comprises
centrifuging the resulting scrubber liquor.
13. The method of claim 9, wherein scrubbing the gas stream with
the second hydrogen peroxide solution further comprises: feeding
the gas stream into a scrubber; feeding the second hydrogen
peroxide solution into the scrubber; removing solids from a
resulting second scrubber liquor; and recycling the second scrubber
liquor into the scrubber with the second hydrogen peroxide
solution.
14. The method of claim 13, further comprising: sampling a mixture
of the second hydrogen peroxide solution and the recycled second
scrubber liquor for a level of hydrogen peroxide; and controlling
the level of hydrogen peroxide in the mixture to be at least a
predetermined level.
15. The method of claim 14, wherein the predetermined level of
hydrogen peroxide is an amount necessary to provide a
stoichiometric excess of hydrogen peroxide for reaction with the
gas stream.
16. The method of claim 9, wherein the chlorine dioxide mixture is
generated in a continuous reaction process concurrently with
oxidizing the gas stream using the chlorine dioxide mixture.
17. The method of claim 16, wherein the chlorine dioxide mixture is
further generated in a process comprising continuously adding a
sodium chlorate solution and a hydrogen peroxide and sulfuric acid
solution to a reaction vessel.
18. The method of claim 17, wherein oxidizing the gas stream using
the chlorine dioxide mixture comprises educting the chlorine
dioxide mixture from the reaction vessel into the gas stream.
19. The method of claim 18, further comprising: passively adding
make-up air to the reaction vessel to maintain the reaction vessel
at ambient conditions while educting the chlorine dioxide mixture
from the reaction vessel.
20. The method of claim 17, wherein oxidizing the gas stream using
the chlorine dioxide mixture comprises pressuring the chlorine
dioxide from the reaction vessel into the gas stream.
21. The method of claim 17, further comprising: adjusting a rate of
addition of the sodium chlorate solution and a rate of addition of
the hydrogen peroxide and sulfuric acid solution to regulate a
generation rate of chlorine dioxide.
22. A system for the removal of at least oxides of nitrogen from a
gas stream, the system comprising: an oxidation tower coupled to
receive the gas stream and a chlorine dioxide mixture; a scrubber
coupled to receive an exit gas stream from the oxidation tower and
coupled to receive a hydrogen peroxide solution and a recycled
liquor from the scrubber; a neutralizer coupled to receive an exit
gas stream from the scrubber; and a chlorine dioxide generation and
injection system for continuous generation of the chlorine dioxide
mixture and injection into the oxidation tower.
23. The system of claim 22, wherein the chlorine dioxide generation
and injection system comprises: a reaction vessel; means for
metering a sodium chlorate solution into the reaction vessel; means
for metering a hydrogen peroxide and sulfuric acid solution into
the reaction vessel; means for diluting the chlorine dioxide
mixture; and means for injecting the chlorine dioxide mixture into
the oxidation tower.
24. The system of claim 23, wherein the means for metering comprise
metering pumps.
25. The system of claim 23, wherein the means for diluting the
chlorine dioxide mixture and means for injecting the chlorine
dioxide mixture into the oxidation tower comprise a passive air
intake to the reaction vessel for providing make-up air to the
reaction vessel and an eductor for pulling the chlorine dioxide
mixture from the reaction vessel and injecting it into the
oxidation tower.
26. The system of claim 23, wherein the means for diluting the
chlorine dioxide mixture and means for injecting the chlorine
dioxide mixture into the oxidation tower comprise a pressurized air
intake to the reaction vessel for diluting and injecting the
chlorine dioxide mixture from the reaction vessel into the
oxidation tower.
27. A system for the removal of at least oxides of sulfur and
oxides of nitrogen from a combustion flue gas, the system
comprising: a water wash coupled to receive a raw flue gas stream;
a first scrubber coupled to receive an exit gas stream from the
water wash and coupled to receive a first hydrogen peroxide
solution; a centrifuge coupled to receive a first scrubber liquor
from the first scrubber and to provide a recycled scrubber liquor
to the first scrubber after removal of solids from the first
scrubber liquor; a control system to maintain a desired hydrogen
peroxide concentration in the recycled scrubber liquor and to
maintain a desired sulfuric acid concentration in the recycled
scrubber liquor; a demister coupled to receive an exit gas stream
from the first scrubber; an oxidation tower coupled to receive an
exit gas stream from the demister, and to receive a chlorine
dioxide mixture; a second scrubber coupled to receive an exit gas
stream from the oxidation tower and coupled to receive a second
hydrogen peroxide solution and a recycled liquor from the second
scrubber; a neutralizer coupled to receive an exit gas stream from
the second scrubber; and a chlorine dioxide generation and
injection system for continuous generation of the chlorine dioxide
mixture and injection into the oxidation tower.
28. The system of claim 27, wherein the chlorine dioxide generation
and injection system comprises: a reaction vessel; means for
metering a sodium chlorate solution into the reaction vessel; means
for metering a hydrogen peroxide and sulfuric acid solution into
the reaction vessel; means for diluting the chlorine dioxide
mixture; and means for injecting the chlorine dioxide mixture into
the oxidation tower.
29. The system of claim 28, wherein the means for metering comprise
metering pumps.
30. The system of claim 28, wherein the means for diluting the
chlorine dioxide mixture and means for injecting the chlorine
dioxide mixture into the oxidation tower comprise a passive air
intake to the reaction vessel for providing make-up air to the
reaction vessel and an eductor for pulling the chlorine dioxide
mixture from the reaction vessel and injecting it into the
oxidation tower.
31. The system of claim 28, wherein the means for diluting the
chlorine dioxide mixture and means for injecting the chlorine
dioxide mixture into the oxidation tower comprise a pressurized air
intake to the reaction vessel for diluting and injecting the
chlorine dioxide mixture from the reaction vessel into the
oxidation tower.
32. The system of claim 27, further comprising: a neutralizer
interposed between the demister and the oxidation tower.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit under 35 U.S.C. 119(e)
of U.S. Provisional Application Ser. No. 60/737,015 filed Nov. 14,
2005, the contents of which are incorporated herein by
reference.
ORIGIN OF THE INVENTION
[0002] The invention described herein was made in part by an
employee of the United States Government and may be manufactured
and used by and for the Government of the United States for
governmental purposes without the payment of any royalties thereon
or therefor.
[0003] 1. Field of the Invention
[0004] The present invention relates generally to emission control
and in particular to the control of emissions from combustion
sources.
[0005] 2. Background of the Invention
[0006] Control of emissions from fossil fuel combustions sources
addresses a major environmental problem. The Environmental
Protection Agency (EPA) through the Clean Air Act regulates the
emissions from fossil-fuel-fired power plants. Initial regulations
were focused on oxides-of-nitrogen (NOx) and oxides-of-sulfur (SOx)
emissions, but newer regulations will include provisions to control
heavy metals (Hg, etc.) and carbon dioxide.
[0007] Gas streams from combustion processes are often scrubbed,
i.e., contacted with water or water solutions, to remove many of
their contaminants. However, these scrubbing processes often
produce hazardous waste streams that must be dealt with.
Furthermore, scrubbing of nitric oxide (NO) is generally
ineffective. To remove this contaminant, the nitric oxide is
typically converted to nitrogen dioxide (NO.sub.2) prior to
scrubbing. Various oxidizing agents have been utilized for this
conversion. However, more effective oxidizing agents are also more
unstable, may not be cost effective, and may be difficult or
dangerous to store.
[0008] For the reasons stated above, and for other reasons stated
below that will become apparent to those skilled in the art upon
reading and understanding the present specification, there is a
need in the art for alternative methods and apparatus for treating
combustion gas streams.
SUMMARY OF THE INVENTION
[0009] Methods and apparatus utilizing chlorine dioxide (ClO.sub.2)
to reduce NOx emissions are described herein. Such methods and
apparatus may be stand-alone systems or may further be incorporated
into more encompassing systems, such as systems utilizing hydrogen
peroxide to reduce SOx, NOx, and mercury or other oxides-of-metal
emissions. The apparatus is modular and components can be added or
removed depending upon the specific requirements for a given
removal operation.
[0010] For one embodiment, the invention provides a method of
treating a gas stream for removal of at least oxides of nitrogen.
The method includes generating chlorine dioxide in a continuous
reaction process, mixing the chlorine dioxide with the gas stream
upon generation, maintaining a desired level of nitric oxide in the
gas stream by regulating a generation rate of the chlorine dioxide,
and scrubbing the gas stream with a hydrogen peroxide solution,
thereby producing a first scrubber liquor. The method further
includes maintaining a desired hydrogen peroxide concentration in
the first scrubber liquor by regulating a flow of the first
hydrogen peroxide solution.
[0011] For another embodiment, the invention provides a method of
treating a gas stream for removal of at least oxides of sulfur and
oxides of nitrogen. The method includes scrubbing the gas stream
with a first hydrogen peroxide solution, thereby producing a first
scrubber liquor, and maintaining a desired hydrogen peroxide
concentration in the first scrubber liquor by regulating a flow of
the first hydrogen peroxide solution. The method further includes
removing a by-product stream containing sulfuric acid after
attaining a desired sulfuric acid concentration in the first
scrubber liquor. The method further includes a demister to reduce
the transport of sulfuric acid mist. The method further includes
oxidizing the gas stream using a chlorine dioxide mixture adapted
to convert nitric oxide to nitrogen dioxide after scrubbing the gas
stream with the first hydrogen peroxide solution and, after
oxidizing the gas stream, scrubbing the gas stream with a second
hydrogen peroxide solution. The method still further oxidizes
elemental mercury to oxides-of-mercury with chloride dioxide and,
after scrubbing the gas stream in a second hydrogen peroxide
solution that contains nitric acid, dissolves the
oxides-of-mercury.
[0012] For a further embodiment, the invention provides a system
for the removal of at least oxides of nitrogen from a gas stream.
The system includes an oxidation tower coupled to receive the gas
stream and a chlorine dioxide mixture, a scrubber coupled to
receive an exit gas stream from the oxidation tower and coupled to
receive a hydrogen peroxide solution and a recycled liquor from the
scrubber, a demister to reduce nitric acid mist, a neutralizer
coupled to receive an exit gas stream from the scrubber that
adjusts the pH to 7.0.+-.2.0, and a chlorine dioxide generation and
injection system for continuous generation of the chlorine dioxide
mixture and injection into the oxidation tower.
[0013] For a still further embodiment, the invention provides a
system for the removal of at least oxides of sulfur and oxides of
nitrogen from a combustion flue gas. The system includes a heat
exchanger to transfer heat from the a raw flue gas stream to the
processed flue gas stream before it enters the flue exhaust stack,
a water wash coupled to receive a raw flue gas stream as it exits
from the heat exchanger, a first scrubber coupled to receive an
exit gas stream from the water wash and coupled to receive a first
hydrogen peroxide solution, a solids removal system coupled to
receive a first scrubber liquor from the first scrubber and to
provide a recycled scrubber liquor to the first scrubber after
removal of solids from the first scrubber liquor, and a control
system to maintain a desired hydrogen peroxide concentration in the
recycled scrubber liquor and to maintain a desired sulfuric acid
concentration in the recycled scrubber liquor. The system further
includes a first demister coupled to receive an exit gas stream
from the first scrubber, an oxidation tower coupled to receive an
exit gas stream from the demister and a chlorine dioxide mixture,
and a second scrubber coupled to receive an exit gas stream from
the oxidation tower and coupled to receive a second hydrogen
peroxide solution and a recycled liquor from the second scrubber.
The system further includes a second demister coupled to receive an
exit gas from the second scrubber. The system still further
includes a neutralizer coupled to receive an exit gas stream from
the second demister coupled to the second scrubber and a chlorine
dioxide generation and injection system for continuous generation
of the chlorine dioxide mixture and injection into the oxidation
tower.
[0014] The invention further includes methods and apparatus of
varying scope.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block schematic of an emission control system in
accordance with an embodiment of the invention.
[0016] FIG. 2 is a schematic of a hydrogen peroxide concentration
control system for use in accordance with one embodiment of the
invention.
[0017] FIG. 3 is a hydrogen peroxide reaction vessel subsystem for
use in accordance with one embodiment of the invention.
[0018] FIG. 4 is a functional schematic of a chlorine dioxide
generation and injection system in accordance with an embodiment of
the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] In the following detailed description of the preferred
embodiments, reference is made to the accompanying drawings that
form a part hereof, and in which is shown by way of illustration
specific preferred embodiments in which the inventions may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to practice the invention, and it
is to be understood that other embodiments may be utilized and that
logical, mechanical, and chemical changes may be made without
departing from the spirit and scope of the present invention. It is
noted that the drawings are not to scale unless a scale is provided
thereon. The following detailed description is, therefore, not to
be taken in a limiting sense, and the scope of the present
invention is defined only by the appended claims and equivalents
thereof.
[0020] Emission control systems in accordance with the invention
address environmental pollutants SOx, NOx, and heavy metals. Such
systems are designed to control emissions of these environmental
pollutants to the levels established by the EPA. This emission
control system provides a method based on hydrogen peroxide to
reduce the SOx, NOx, and metal and metal oxide emissions from
combustion sources to acceptable levels as established by the EPA.
In addition, useful by-product streams of sulfuric acid, nitric
acid, salts of these acids, and feedstock for oxides-of-metal
production may be isolated.
[0021] FIG. 1 is a block schematic of an emission control system in
accordance with an embodiment of the invention. The process starts
with a gas stream, such as raw flue gas 150 after the particulates
have been removed. There are several options for this design and
steps can be omitted or alternate unit operations may be
substituted for the general processes depending on the requirements
of the installation. These alternate steps are noted during the
description of the process.
[0022] The first step of the process is to use a cooling and wash
chamber 102 to cool and remove some of the particulates in the
entering flue gas 150. Process water from chamber 103 is provided
to wash chamber 102. Particulates, wash water and heat are sent
from chamber wash 102 to the sedimentation and cooling pond 104.
Water is then re-circulated from pond 104 back to water chamber
103.
[0023] The washed flue gas is fed from wash chamber 102 to a
scrubber tower 105 for the removal of SOx and/or heavy metals.
Scrubber tower 105 uses hydrogen peroxide from hydrogen peroxide
storage 112 to oxidize sulfurous acid (H.sub.2SO.sub.3) to sulfuric
acid (H.sub.2SO.sub.4) to prevent reemission of SO.sub.2. Hydrogen
peroxide storage 112 preferably provides aqueous hydrogen peroxide
of approximately 35 percent to 70 percent by volume, and more
preferably at approximately 70 percent by volume. As the scrubber
liquor pH decreases due to the formation of sulfuric acid, most of
the heavy metal oxides, including Hg, etc., are dispersed as metal
oxides and/or dissolved hydroxides are converted to sulfates. The
remaining un-dissolved particulates and insoluble sulfates are
removed with a solids removal system 106, e.g., a solid-bowl
centrifuge, a continuous belt filter, or other system for the
continuous removal of solids from a liquid stream. FIG. 1 depicts
the use of a centrifuge 106 as the solids removal system as but one
example.
[0024] Centrifuge 106 continuously removes the solids and
circulates the scrubber liquor through valve 109 back to scrubber
tower 105 for continuous scrubbing and cleaning the flue gas. When
the scrubber liquor (sulfuric acid) reaches the desired
concentration, the cleansed scrubber liquor is discharged from the
centrifuge 106 through valve 110 then drained to sulfuric acid
storage 111, which can then be utilized for fertilizer or
industrial uses. The solids from centrifuge 106 are discharged to
the recycle solids container 107. Soluble metals and metal oxides
present in the cleansed scrubber liquor may further be removed as
insoluble solids upon neutralization of the sulfuric acid as may be
performed, for example, during the production of fertilizer.
[0025] The concentration of hydrogen peroxide in the first
scrubbing mixture is maintained at a predetermined level, e.g., 0.1
to 5 percent by volume. Concentration of the first scrubbing
mixture may be maintained using a hydrogen peroxide controller of
the type described below. Additional detail of such a controller
may be found in U.S. Pat. No. 6,039,783 issued Mar. 21, 2000, to
Lueck, et al. and U.S. Pat. No. 6,641,638 issued Nov. 4, 2003, to
Lueck, et al.
[0026] When the flue gas exits the scrubber tower 105 it contacts
the demister 108, where the mist that contains sulfuric acid
coalesces. The coalesced mist is returned to the scrubber tower 105
and the desulfurized flue gas flows to the neutralizer 118, if only
the SOx system is used. If NOx is present, the neutralization is
bypassed and the flue gas flows directly to the oxidation tower
113. Residual acid gases are neutralized with a base in neutralizer
118, the cleansed and neutralized flue gas passes through the heat
exchanger, and then exits through the exhaust stack. Alternately,
an exhaust fan could be used in place of the heat exchanger at the
outlet of the neutralizer 118, if desired, to boost the cleansed
and neutralized flue gas out of the exhaust stack as processed flue
gas 152.
[0027] The analytical system used for the control system, as
initially described in U.S. Pat. No. 6,039,783, contains a unique
process that controls the concentration of hydrogen peroxide. The
process is controlled by a programmable logic controller (PLC)
designed to measure the concentration of hydrogen peroxide and to
add additional hydrogen peroxide as required to maintain the preset
concentration. In addition to the hydrogen peroxide controller, the
control system contains two commercial controllers, one for pH and
the other to measure the concentration of sulfuric acid, nitric
acid, or salts of these acids. These commercial controllers are
used to maintain a target pH or concentration and to add reagents
as required. Details of the design and operation of the control
system are given in the following section.
[0028] The block flow diagram for a hydrogen peroxide controller
suited for use with embodiments of the invention is shown in FIG.
2. The hydrogen peroxide PLC that controls the operations starts
the first sequence by pulling a sample into the system from sample
point 231 with pump 232 and pumping it through valve 233. The pH
probe 247 and conductivity probe 248 are exposed to the sample
before passing the sample into metallic filter 234 where a portion
of the sample passes through the filter 234 and the remainder
bypasses the filter 234 and washes the residues back to sample
return 245. The filtrate that passed through filter 234 continues
through valve 235, multiport valve 236, and sample loop 237. From
sample loop 237 the sample returns to multiport valve 236 and then
exits and returns back to sample return 245. The sequence of events
described above serves to collect a sample from the sample point
231, filter the sample, fill the sample loop that has a known
volume, and return the unused sample to sample return 245. While
the sample loop 237 is being filled, the metering pump 238 pulls
sodium hypochlorite from container 239 and injects a known volume
into reaction cell 240 through multiport valve 236. The second
sequence is triggered by the PLC, which sends a signal to rotate
multiport valve 236 and pump a second quantity of sodium
hypochlorite, but this time it is used to displace the sample from
the sample loop 237, which is pumped into the reaction cell 240.
The reaction of sodium hypochlorite with hydrogen peroxide produces
water, sodium chloride, and oxygen, which causes an increase in
pressure in the reaction cell that is sensed by the pressure
transducer 241. Calibration data programmed into the PLC for
pressure change as a function of hydrogen peroxide concentration is
used to control the concentration of hydrogen peroxide in the
system. If the measured concentration is below the set
concentration, a pump is activated to transfer hydrogen peroxide
from storage. The hydrogen peroxide pump stops when the measured
concentration is greater than the set concentration. This cycle is
repeated continuously to maintain the set concentration of hydrogen
peroxide. While the concentration is being measured, the filter 234
is back-flushed with water 243 through valve 244 to remove
particulates captured by the filter 234. When the back-flush
starts, valve 233 closes and pump 232 stops and the water 243
pushes through the filter 234 and back to the sample return 245. In
addition, the contents of the reaction cell may be expelled through
valve 242 to sample waste 246 at this time. Once the back-flush is
complete, the PLC returns the process to the first sequence and
sampling process start over again.
[0029] FIG. 3 shows the details of the reaction cell 240. The
reaction cell body 351 has an internal chamber 352 that is attached
to a gauge guard 353 that protects the pressure transducer 241.
Sodium hypochlorite and the sample are pulled through sodium
hypochlorite inlet 355 by metering pump 238. Once the reaction in
the pressure cell is complete, it is discharged through valve 242
to sample waste 246. The reaction cell is fabricated preferably
from a machinable corrosion-resistant polymer. While the foregoing
brief description of the control of hydrogen peroxide concentration
is included to aid the reader, a more detailed description is
provided in U.S. Pat. No. 6,039,783 and U.S. Pat. No. 6,641,638 as
noted previously.
[0030] The pH may be controlled with commercially-recognized
devices, such as Rosemont's model 0054pH/ORP-08 pH controller and a
model 306P-02010055 pH probe. The pH probe is item 247 in FIG. 2
and it is used to continuously measure the pH when the acid is
neutralized to produce a fertilizer. The pH controller system has
proportional algorithms that adjust the pump feed rate as the pH
set-point is approached. For one embodiment, the pH is controlled
to a level of between 7.0 and .+-.2.0 by adding a base, e.g.,
potassium hydroxide. The concentration of sulfuric acid and/or
nitric acid, and salts of these acids, is controlled with the
Rosemont model 1054B%1-99 controller. The conductivity probe model
228-02-21-54-61 is item 248 in FIG. 2. Once calibrated for the
specific ion used in the system, the proportional control
algorithms adjust pumping rate for the base used to form the
salts.
[0031] Returning to FIG. 1, for removal of NO.sub.x emissions, the
neutralized flue gas from neutralizer 118 flows to NO oxidation
tower 113, where nitric oxide (NO) is oxidized to nitrogen dioxide
(NO.sub.2). Alternatively, the flue gas may be passed directly from
the demister 108 to the NO oxidation tower 113 for later
neutralization, eliminating neutralizer 118. Chlorine dioxide
(ClO.sub.2) from chlorine dioxide feed system 114 is fed into NO
oxidation tower 113.
[0032] The chlorine dioxide fed to oxidation tower 113 reacts with
NO to convert it to NO.sub.2 and HNO.sub.3 as follows:
2NO+ClO.sub.2+H.sub.2O.fwdarw.NO.sub.2+HCl+HNO.sub.3
Both the HCl and HNO.sub.3 may be removed by subsequent
scrubbing.
[0033] The oxidized NO in the flue gas flows from the oxidation
tower 113 to the scrubber tower 115 where it is captured as nitric
acid in an acidic hydrogen peroxide scrubber liquor generally as
follows:
2NO.sub.2+H.sub.2O.sub.22HNO.sub.3
[0034] The concentration of hydrogen peroxide in the second
scrubbing mixture ranges from 0.1 percent to 5 percent by volume
and is controlled by a second hydrogen peroxide controller
determining make-up hydrogen peroxide from the hydrogen peroxide
storage 112 to add to the re-circulating scrubber liquor from pump
116. The cleansed flue gas that exits from scrubber tower 115
passes into neutralizer 119, where base is added to neutralize any
residual acid gases by adjusting the pH to 7. Once neutralized, the
cleansed flue gas exits through the heat exchanger, and then to the
exhaust stack as processed flue gas 152. Alternately, an exhaust
fan could be used in place of the heat exchanger at the outlet of
the neutralizer 119, if desired, to boost the cleansed and
neutralized flue gas out of the exhaust stack as processed flue gas
152. The mixed acid solution containing nitric acid and
hydrochloric acid is then sent to mixed acid storage 117 for
recovery or disposal.
[0035] Although chlorine dioxide is recognized as an unstable gas,
the various embodiments generate the chlorine dioxide gas at the
time of use and, optionally, providing for concurrent dilution with
air. Chlorine dioxide may be produced continuously by mixing a
solution of hydrogen peroxide (H.sub.2O.sub.2) and sulfuric acid
(H.sub.2SO.sub.4) with the solution of sodium chlorate
(NaClO.sub.3). The produced chlorine dioxide may be mixed, as it is
formed, with air to keep the gas phase concentration below 10
percent.
[0036] FIG. 4 is a functional schematic of a chlorine dioxide feed
system 414 in accordance with an embodiment of the invention. The
chlorine dioxide is generated through a reaction of a sodium
chlorate solution 460 and a hydrogen peroxide and sulfuric acid
solution 462. The sodium chlorate solution 460 is metered into a
reaction vessel 468 using a metering pump 464. The hydrogen
peroxide and sulfuric acid solution 462 is metered into the
reaction vessel 468 using a metering pump 466.
[0037] For one embodiment, the hydrogen peroxide and sulfuric acid
solution 462 is produced using 37.6 wt % of sulfuric acid (98 wt %)
and 7 wt % of hydrogen peroxide (50 wt %) in water. The solution
can be prepared by slowly adding sulfuric acid (98 wt-%) to water,
allowing that to cool, and then adding hydrogen peroxide (50 wt-%).
For a further embodiment, the sodium chlorate solution 460 contains
50 wt % sodium chlorate (NaClO.sub.3) in water. A solution having a
specific gravity of 1.4 may be prepared by mixing equal weights of
NaClO.sub.3 and water. The solution can be prepared by slowing
adding sodium chlorate to water and stirring until the solids are
dissolved.
[0038] The reaction vessel 468 is preferably of a material
resistant to the oxidizing power of the reagents, e.g.,
glass-lined, polyethylene-lined, Teflon.RTM.-lined, etc., and
equipped with a mixer 470. To initiate the reaction, it may be
desirable to add crystalline sodium chlorate to some hydrogen
peroxide and sulfuric acid solution 462 in the reaction vessel 468,
and then meter in further hydrogen peroxide and sulfuric acid
solution 462 and sodium chlorate solution 460 after the reaction
has begun, e.g., after approximately a 5 minute delay. By metering
in at approximately stoichiometric molar quantities of the reagents
460 and 462, a desired production rate of chlorine dioxide may be
maintained.
[0039] For one embodiment, desired rate for production of chlorine
dioxide is an amount sufficient to reduce NO emissions from a flue
gas stream 484 to a desired level when mixed with the flue gas 484
as described below. The reaction of the reagent solutions 460 and
462 may proceed generally as follows:
2NaClO.sub.3+H.sub.2SO.sub.4+H.sub.2O.sub.2.fwdarw.2ClO.sub.2+O.sub.2+Na-
.sub.2SO.sub.4+2H.sub.2O
[0040] Due to dilution as the reaction proceeds, the reaction
vessel 468 may need to be drained occasionally for the removal of
excess water and sodium sulfate (Na.sub.2SO.sub.4).
[0041] The reaction vessel may further include an air intake 472 to
maintain the reaction vessel 468 at approximately ambient pressures
and to provide an air sweep for dilution of the produced chlorine
dioxide. For one embodiment the air intake 472 is passive and
provides make-up air as a chlorine dioxide/air mixture 474 is
pulled from head space of the reaction vessel 468 by an eductor
476. Alternatively, the air intake 472 may include regulated
pressure feed to push the chlorine dioxide/air mixture 474 out of
the reaction vessel 468. If air intake 474 is not passive, the
eductor 476 may be eliminated. Note that while air is used in this
example, an inert gas may also be substituted.
[0042] The eductor 476 is operated using a pressurized air feed
478, resulting in a further diluted chlorine dioxide/air mixture
480 for feed into flue 482. Note that the flue may represent the
oxidation tower 113 of FIG. 1. In this example, the incoming flue
gas 484 would represent the output of the neutralizer 118 or the
demister 108 of FIG. 1, and the outgoing flue gas 486 would
represent the output of the oxidation tower 113 fed to scrubber
115.
[0043] The particulate material in the SOx scrubber liquor is a
mixture of fly-ash and insoluble sulfates that include mercury and
other heavy metals. These solid materials are removed from the
scrubber sump with a solids removal system, such as a continuous
filter, centrifuge, or a combination of a continuous filter and a
centrifuge. To further improve the separation efficiency, the
scrubber sump can be divided into two compartments. The first
compartment is designed to receive the scrubber liquor that is
returning from the scrubber tower and the second compartment is
designed to receive the cleansed scrubber liquor and the overflow
from the first compartment. The scrubber pump draws the scrubber
liquor from the middle of the second compartment and the continuous
filter or centrifuge draws from the bottom of the first
compartment. This two compartment configuration can facilitate
maximizing the concentration of particulates going to the filter or
centrifuge and minimizing the particulates going to the scrubber
pump.
[0044] Methods and apparatus for controlling emissions have been
described. Some methods utilize hydrogen peroxide to reduce SOx and
mercury (or other oxides-of-metal) emissions prior to treatment to
reduce NOx. For removal of NO, chlorine dioxide is generated
continuously and fed into the gas stream to be treated. By
utilizing methods and apparatus in accordance with the invention,
combustion flue gas streams can be treated for the removal of NOx,
as well as SOx and oxides-of-metal, while isolating useful
by-products streams of nitric acid, sulfuric acid, salts of nitric
acid, salts of sulfuric acid and solids for the recovery of the
heavy metals. One of the significant advantages of the present
invention is the fact that the process can be run continuously,
with measuring and adjustments made in real time while the process
is being performed. Computer monitoring can initiate flow changes
of reagents in response to automatic measurements to maintain
desired process conditions.
[0045] Although specific embodiments have been illustrated and
described herein, it will be appreciated by those of ordinary skill
in the art that any arrangement that is calculated to achieve the
same purpose may be substituted for the specific embodiments shown.
This application is intended to cover any adaptations or variations
of the present invention. Therefore, it is manifestly intended that
this invention be limited only by the claims and the equivalents
thereof.
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